1. Field of the Invention;
The present invention relates to a multiple-band digital frequency synthesizer receiver. More specifically, the present invention relates to a multiple-band receiver employing a digital frequency synthesizer employing a phase locked loop.
2. Description of the Prior Art;
A typical prior art radio receiver comprises as a local oscillator for a tuner having a parallel resonance circuit comprising a coil and a capacitor, in which the inductance of the coil or the capacitance of the capacitor is varied to obtain a desired tuning frequency. However, such a local oscillator is liable to suffer fluctuations of the oscillation frequency caused by temperature characteristics of the coil, capacitor and the other oscillator components. Thus, it is rather difficult to obtain a stabilized oscillation frequency.
A digital frequency synthesizer has also been proposed and is in practical use. Such a frequency synthesizer is much more advantageous in that it can provide a much more stable oscillation frequency. A typical frequency synthesizer employs a phase locked loop, which is often simply referred to as "PLL".
A frequency synthesizer employing a phase locked loop usually comprises a voltage controlled oscillator, the oscillation frequency of which is controllable as a function of an output voltage, which is low pass filtered, obtainable from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of an output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at a frequency division rate which is adapted to be variable as a function of a control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider. Therefore, if such a voltage controlled oscillator is used as a local oscillator of a tuner of a radio receiver, automatic scanning of the receiving band can be effected by varying the frequency division rate of the programmable frequency divider, as described above. If and when the frequency of a broadcasting station is tuned by the tuner, an output is obtained by the receiver, which is utilized to disable the variation of the frequency divider control signal.
The variable range of the frequency division rate of the programmable frequency divider should be determined depending on the range of the local oscillation frequency of the local oscillator for a given receiving band, such as an AM medium wave band, AM short wave band, FM band, or the like, and the frequency difference between the two adjacent broadcasting station frequencies, such that the receiver can receive any broadcasting frequency of a given receiving band. Thus, it is appreciated that the variable range of the frequency division rate of the programmable frequency divider should be different depending on the receiving band. For example, the Japanese standard for FM broadcasting has been determined as the range of receiving frequencies being 76 through 90 MHz, the intermediate frequency being 10.7 MHz, and an inter-station frequency being 100 KHz. This means that the local oscillation frequency should be varied 65.3 through 79.3 MHz, assuming that a lower sideband superheterodyne is employed. Accordingly, it is necessary that the frequency division rate of the programmable frequency divider be varied from 653 to 793. On the other hand, the Japanese standard for AM medium wave broadcasting has been determined as the range of receiving frequencies being 535 through 1605 KHz, the intermediate frequency being 455 KHz and the inter-station frequency being 10 KHz. This means the local oscillation frequency should be varied from 990 to 2060 KHz and accordingly the frequency division rate of the programmable frequency divider need be varied from 99 to 206, assuming that an upper sideband superheterodyne is employed.
Assuming that the receiving band is switched from a state of receiving FM broadcasting to a state of receiving AM broadcasting, the difference between the variable range of the frequency division rate of the programmable frequency divider for reception of FM broadcasting and the variable range of the frequency division rate of the programmable frequency divider for reception of AM broadcasting makes impossible the reception of AM broadcasting because of the receiving frequency in such a situation being outside of the range of the receiving band of AM broadcasting, if and when a receiver designed for reception for FM broadcasting is simply utilized for reception of AM broadcasting, and vice versa. Thus, in implementing a multiple-band radio receiver, it is necessary to provide a multiple number of frequency synthesizers each of which is adapted for reception of the corresponding broadcasting band. However, provision of a multiple number of frequency synthesizers complicates the structure of the receiver. It is desired that a multiple-band receiver with simplified structure be provided. Further it is desired that a multiple-band receiver be provided in which automatic scanning of the receiving band is effected from either the minimum or the maximum frequency of the receiving band when the receiving band is switched.
Assuming that there is provided an improved multiple-band receiver employing a digital frequency synthesizer which is capable of being switched to a plurality of receiving bands, another problem arises in connection with display of the broadcasting frequency to be received using the digital data obtainable from the said digital frequency synthesizer. As well known to those skilled in the art, in a superheterodyne receiver employing a typical digital frequency synthesizer employing a phase locked loop, the local oscillation frequency of the voltage controlled oscillator is selected to be higher or lower than the frequency of the broadcasting frequency to be received by the frequency difference commensurate with the intermediate frequency of the radio receiver, while the frequency division rate of the programmable frequency divider is varied so as to be proportional to the required local oscillation frequency of the voltage controlled oscillator. Hence, it is appreciated that the frequency to be received is proportional to the frequency division rate of the programmable frequency divider, with the frequency difference commensurate with the intermediate frequency. Because of this frequency difference, the frequency division rate of the programmable frequency divider can not be used directly for the purpose of displaying the frequency of the broadcasting signal to be received, in spite of the fact that the frequency division rate is of a digital nature and thus is easy for digital processing in general. Thus, it is desired that a multiple-band receiver be provided in which it is easy to display the frequency of a broadcasting signal to be received using the digital data obtainable from the digital frequency synthesizer.
Another problem encountered in connection with a multiple-band digital frequency synthesizer receiver is that the intermediate frequency is different depending on the receiving bands and in addition the intermediate frequency may be different even for the same receiving band, as the broadcasting standards including the intermediate frequency are different depending on the territory where the receiver is used. Thus, it is further desired that a multiple-band digital frequency synthesizer receiver be adapted to select a desired intermediate frequency in response to the band switching, thereby to compensate for the difference in the intermediate frequency caused depending on the receiving band, the broadcasting standard, and the like.
A further problem encountered in a superheterodyne radio receiver employing a digital frequency synthesizer is that the inter-station frequency, i.e., the frequency difference of adjacent broadcasting stations, is fixed which result in a dilemma that reception becomes impossible if the inter-station frequency is changed by way of revision of the broadcasting standard. Thus, it is desired that such inter-station frequency be achieved by such a frequency synthesizer receiver can be adaptably changed with ease.